Over the past few years, "superalloys" have been utilized to a greater extent, and production of these metals has increased, because of the need for articles having the unique properties of these materials. Among these properties are high temperature strength, high temperature oxidation resistance and corrosion resistance. With the increase in use of these superalloys, the amount of scrap and other waste metals containing them grows as well. The superalloy scrap can include such products as turnings, grindings, mold gates, flashings, etc., in addition to used metal parts.
Superalloys are commonly comprised of nickel, cobalt and chromium, with lesser amounts of molybdenum, tungsten, aluminum, iron, and niobium as well. The potential recovery of large amounts of nickel, cobalt, and chromium thus makes recovery of metal values from the scrap very important, as these elements can be reused in the making of more superalloy material.
However, superalloy scrap has generally presented various problems with regard to recovery of these elements. In some cases superalloy scrap has been downgraded and, sold for its nickel content with the subsequent loss of the other alloying elements or similar disposal steps have been taken, because the individual metal values cannot be isolated for reuse. Several other methods of recovering metal values from such scrap have been attempted, but these methods all have certain drawbacks.
One method, as disclosed in U.S. Pat. No. 3,649,487, is to melt the metal scrap along with non-metallic substances, such as silicon and carbon, form the melt into an anode, and electrolyze the anode in a neutral or acid bath. This method requires complex processing of the scrap, results in loss of some of the elements, and is hard to make into a continuous process. Another method employed, as disclosed in U.S. Pat. No. 3,607,236 and 4,442,073, involves placing the superalloy scrap in an acidic aqueous solution and extracting the metal components through various means. This method suffers from the drawback that a specific oxidation state must be reached for proper extraction of most of the metal elements involved, and loss of some elements occurs through improper oxidation. Finally, dissolution of the scrap in molten aluminum has been employed, as disclosed in U.S. Pat. No. 2,946,677, but this process adds impurities to the metals recovered and is also hard to make continuous. It is thus desirable to have a process which can maximize recovery of metal values from superalloy scrap, and which can be run continuously as well.